Definitely have this problem, or a similar one - DD1 leans sideways out of the seatbelt to get her arm out and then wears it as a lap belt. I know another SN kid who does exactly the same.
The solution depends on your car a bit. There is a 5-pt harness made by crelling harnesses which will go over the entire shebang, ie attaches behind the seat of the car itself and goes round both seat of the car and child seat. BUT it only fits your car if there is a way of passing the straps through between the base of the seat of the car and the back of the seat of the car. No good for us, because the two are fused to make it easier to put the back seat down for carrying furniture and stuff. Google Crelling anyway, they're very helpful on the phone.
What we have done is to gerry rig a very small additional strap; it does not restrain the child or bear any weight at all. It just holds the seatbelt in place on the shoulder. But I don't know if my heath robinson solution would solve your problem too.
Fwiw:
I got a pack of toy straps which have a loop at one end and a popper fastening at the other. I attached something solid to the loop end (in our case a plastic linky from a baby toy but whatever works) and passed the strap through the guiding slots on the unused side of the booster seat. So the solid object is behind the seat, holding the strap in position, and the strap dangles down by their shoulder. Fasten the seatbelt as usual and then popper the toy strap around it. It makes it impossible to pull the strap down, or very far out, and it stops you from leaning away from the seatbelt side. However, in case of accident, you can easily unpopper the strap and get the child out. I think it's fairly safe. It's definitely waaaay safer than driving along half out of your seatbelt.
Plan c) is to get a specific seat with a built-in 5 point harness. As 2CD says there are hybrid seats which are designed for you to take the 5 point harness off at some point and as he is small that might last you for a bit. There are specific SN seats with 5 point harnesses which go up to much larger sizes, BUT these SN seats are usually built for kids with mobility and support issues and as a result are heavy, bulky and cost around £500.